K9s are good trucks apart from the cab rust, but that's true for most '50s vehicles. They're rather smaller than an RL - nominally a 1 ton truck, and actually gross a bit over 6 tonnes. 4 litre petrol engine will get you around 15 - 18 mpg unless you have a heavy right foot. Crash gearbox, but quite a decent one. Good off road in my experience.
Alternatively if you go more modern there's the Bedford MK / MJ and the Leyland DAF 4x4, though you're unlikely to get the latter at least within your budget. For me the MK / MJ are a bit underpowered; the 330's a decent engine but isn't particularly torquey.
Early Unimog 400 series might be a possibility, I'm not sure what soft cab petrol 404s make now but they used to be very cheap. Very good trucks. Morris Commercial MRA1 is equivalent to the K9 but much more primitive, can be picked up cheap if they come up. 1 ton Humber is a good truck but has some weak spots, is stupidly thirsty by comparison and I suspect a decent one will be beyond your budget - a poor one is unlikely to be much more than a pile of red dust on top of a pair of axles. Commer Q4s look good but IMO not a patch on an RL. A bargain basement Renault / Dodge RB44 might be a possibility as mentioned earlier, though I've spent far too many hours trying to sort out the brakes on Dodge trucks to feel enthusiastic about them.